<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>

    <style>
        body {
            margin: 0px;
        }
        div {
            padding: 20px;
            margin: 50px;
        }

        div.border1 {
            border: 1px solid black;
        }
        div.border2 {
            border-bottom: 1px solid red;
        }
        div.border3 {
            border: 1px solid lightgray;
            border-radius: 10px;
        }
        div.border4 {
            background-color: lightblue;
            border-left: 10px double gray;
        }
    </style>
</head>
<body>
    <div class="border1">
        在四边都有边框
    </div>
    <div class="border2">
        红色底部边框
    </div>
    <div class="border3">
        圆角边框
    </div>
    <div class="border4">
        左侧边框带宽度，颜色为蓝色
    </div>
</body>
</html>